(1) Field of the Invention
The present invention relates to a secondary cell having lithium as an active material and non-aqueous electrolyte or the like, more particularly, to a new material for an electrode thereof.
(2) Description of the Related Art
Recently, researches of secondary cells having lithium as an active material has been actively pursued as they show great promise due to their high energy density.
A success or a failure of the study depends on performance of the negative electrode of the secondary cell. Conventionally, lithium metal, lithium alloy and the like are utilized for the negative electrode.
In a secondary cell having a lithium metal negative electrode, its low discharge efficiency, i.e. 50% more or less, makes it difficult to obtain efficient charge/discharge cycle characteristics. Also in the course of the charge/discharge cycles, dendrites crystallize on the surface of lithium, causing an internal short circuit. In a cell having a lithium alloy negative electrode, pulverization of lithium alloy occurs in the course of the charge/discharge cycles, which makes it difficult to improve the charge/discharge cycle characteristics remarkably.
A secondary cell having a negative electrode including intercalation compounds of graphite and lithium is proposed in Japanese Patent Publication No. 57-208079. In such a cell, lithium ions are absorbed into carbon's crystalline structure, thereby retaining substantially 100% charge/discharge efficiency as well as circumventing the problem of the pulverization. However, because such a cell has only a small capacity, a method of its enlargement for high energy density is still sought.
Heat-treated cokes or polymers have recently attracted considerable attention. For instance, in Japanese Laid-open Patent Application No. 63-121259, a property of the cell was discussed by studying some properties of cokes such as density, crystallite size in the direction of c axis Lc and specific surface area S. In repetition of the tests, it was learned that a correlation between the density and the spacing of (002) planes d.sub.002 was also an important factor besides the aforementioned three properties when enlarging the cell capacity.
In Japanese Laid-open Patent Application No. 63-114056, a cell having heat-treated polymers was discussed by studying the correlation among a H/C specific, the crystallite size in the direction of c axis Lc, and the spacing of (002) planes d.sub.002. However, the discussion did not lead to a definite correlation between the density and the spacing of (002) planes d.sub.002, therefore, a method to enlarge the cell capacity was not successfully disclosed therein.
In summary, the conventional secondary cells having lithium as the active material retain an insufficient capacity at the negative electrode, making it impossible to produce a secondary cell with an increased capacity.